<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div class="mainBox">
    <div class="head">New Game</div>
    <div class="body">
        <div class="title">Select a number</div>
        <div class="content">
            <input placeholder="Between 10 to 20" type="number" min="10" max="20">
            <div class="selectors">
                <select>
                    <option value="123">123</option>
                    <option value="456">456</option>
                </select>
            </div>
        </div>
    </div>
</div>
</body>

<style>
    /*输入框被点击时*/
    .body .content input:focus{
        /*外边框*/
        outline: 0;
        /*border: solid 1px red;*/
        /*box-shadow:0 1px 5px rgba(0,123,255,.25);*/
        box-shadow:0 0 0 0.2rem rgba(0,123,255,.25);
    }
    .body .content select:focus{
        /*外边框*/
        outline: 0;
        /*border: solid 1px red;*/
        box-shadow:0 0 0 0.2rem rgba(0,123,255,.25);
    }
    *{
        font-size: 16px;
        font-weight: 400;
        line-height: 1.5;
        box-sizing: border-box;
        background-color: #f8f9fa;
    }
    .mainBox{
        width: 463px;
        height: 400px;
        margin: 0 auto;
        border: solid 1px rgba(0,0,0,.125);
        background-color: #fff;
        border-radius: .25rem;
    }
    .head{
        height: 50px;
        border:solid 1px;
        display: flex;
        flex-direction: column;
        justify-content: center;
        align-items: center;
        background-color: #343a40!important;
        color: white;
        font-weight: 700!important;
    }
    .body{
        padding: 1.25rem;
        background-color: #fff;
    }
    .body .title{
        /*border: solid 1px blue;*/
        margin-bottom: 5px;
        background-color: #fff;
    }
    .body .content input{
        border: solid 1px #ced4da;
        border-radius: .25rem;
        padding: .375rem .75rem;
        width: 100%;
    }
    .body .selectors{
        margin-top: 1rem;
    }
    .selectors select{
        width: 100%;
        border: solid 1px #ced4da;
        border-radius: .25rem;
        padding: .375rem .75rem;
    }
</style>
</html>